1. Field of the Invention
The present invention relates to a method for forming contact holes in a semiconductor device, and more particularly, to a method for forming contact holes having different depths.
2. Background of the Related Art
In a semiconductor device, a plurality of regions or layers are formed that are subsequently interconnected to create components and circuitry. The interconnection between one conductive layer and another conductive region through an insulating layer such as a silicon oxide layer is known as a contact. This contact is made by inserting a conducting material into a previously made contact hole. A contact hole is formed by selectively etching the insulation layer. Specifically, a photoresist is first applied as a thin film to the insulting layer and subsequently exposed through a mask. The mask contains clear and opaque features that define the pattern of a contact hole. The area of the photoresist film which is exposed to light is soluble (in the case of a positive, photoresist) and the exposed portion is removed in a subsequent development process. The remainder of the photoresist film acts as an etch-mask. A succeeding etching step removes the exposed part of the insulating layer but does not remove the insulating layer located under the photoresist mask. This results in the formation of a contact hole.
It is also known to etch a plurality of contact holes having different depths.
FIG. 1 through FIG. 8 are cross-sectional views of intermediate structures formed during a conventional process of forming contact holes having different depths.
FIG. 1, illustrates a semiconductor substrate 100, a first conductive layer 101 and a second conductive layer 102. The thickness of the portion of insulating layer 103 that cover first conductive layer 101 is different from the thickness of the portion of insulating layer 103 that covers the second conductive layer 102. Thus, the contact hole required for exposing first conductive layer 101 has a different depth from that required for exposing second conductive layer 102.
Referring to FIG. 2, photoresist film 104 is applied to insulating layer 103 and then is developed through mask pattern 105, which is typically applied over the photoresist film 104 with some degree of misalignment. Mask pattern 105 may be constituted by a glass 105A which is selectively coated with chromium 105B into which there is opened a window 105C.
As shown from FIG. 3, through window 105C, the exposed part 106 of photoresist film 104 is removed by a solution. As a result, photoresist film 104 is patterned to form an etching mask. Therefore, as shown in FIG. 4, part of insulating layer 103 located over first conductive layer 101, is also etched, with the same degree of misalignment, resulting in the formation of a first contact hole 107.
Thereafter, in order to form a second contact hole, another photoresist film 108 is applied to the surface of insulating layer 103 having a first contact hole 107 etched as depicted in FIG. 5. Then, another mask pattern 109 is laid over photoresist film 108, which typically exhibits misalignment in another direction. The mask pattern 109 is similarly made by a mask substrate 109A such as glass which is covered with chromium 109B into which there is opened a window 109C.
As shown from FIG. 6, the exposed part 110 of the photoresist film 108 is developed to form an etching mask for the second contact hole. Subsequent etching transfers the etching mask pattern to insulating layer 103, resulting in a second contact hole 111, as shown in FIG. 7. Thus, with the conventional method of forming multiple contact holes such as first contact hole 107 and second contact hole 111, the contact holes are formed and have differing misalignments.
Thus, when a metallization process as shown in FIG. 8 is performed, metal wirings 112 and 113, intended to be located in alignment with contact holes 107 and 111, have a considerable step coverage deficiency due to the misalignments of the contact holes.